Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus is provided that has a sensor at substrate level, the sensor including a radiation receiver, a transmissive plate supporting the radiation receiver, and a radiation detector, wherein the sensor is arranged to avoid loss of radiation between the radiation receiver and a final element of the radiation detector.

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 10/924,202, filed Aug. 24, 2004, which claimspriority from European patent application EP 03255395.0, filed Aug. 29,2003, each application incorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective numerical aperture (NA) of thesystem and also increasing the depth of focus.) Other immersion liquidshave been proposed, including water with solid particles (e.g. quartz)suspended therein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

A number of sensors are typically used at substrate level for evaluatingand optimizing imaging performance. These may include a transmissionimage sensor (TIS), a spot sensor for measuring exposure radiation doseand an integrated lens interferometer at scanner (ILIAS). The TIS andILIAS are described below.

A TIS is a sensor that is used to measure the position at substratelevel of a projected aerial image of a mark pattern at the mask(reticle) level. The projected image at substrate level may be a linepattern with a line width comparable to the wavelength of the exposureradiation. The TIS measures these mask patterns using a transmissionpattern with a photocell underneath it. The sensor data may be used tomeasure the position of the mask with respect to the substrate table insix degrees of freedom (three in translation and three in rotation). Inaddition, the magnification and scaling of the projected mask may bemeasured. Since the sensor is typically capable of measuring the patternpositions and influences of all illumination settings (sigma, lens NA,all masks (binary, PSM, etc.)) a small line width is preferable. The TISmay also be used to measure the optical performance of the lithographicapparatus. Different illumination settings are used in combination withdifferent projected images for measuring properties such as pupil shape,coma, spherical aberration, astigmatism and field curvature.

An ILIAS is an interferometric wavefront measurement system that mayperform static measurements on lens aberrations up to a high order. Itmay be implemented as an integrated measurement system used for systeminitialization and calibration. Alternatively, it may be used formonitoring and recalibration “on-demand”.

SUMMARY

In systems with high NA and in particular in liquid immersion systems,conventional sensors at substrate level may suffer poor sensitivity.

Accordingly, it would be advantageous, for example, to provide a sensorat substrate level with high sensitivity and which is suitable for usein a high NA system.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

-   -   an illumination system configured to condition a radiation beam;    -   a support constructed to hold a patterning device, the        patterning device being capable of imparting the radiation beam        with a pattern in its cross-section to form a patterned        radiation beam;    -   a substrate table constructed to hold a substrate;    -   a projection system configured to project the        patterned-radiation beam onto a target portion of the substrate;        and    -   a sensor at substrate level comprising a radiation receiver, a        transmissive plate supporting the radiation receiver, and a        radiation detector, the sensor being arranged to avoid loss of        radiation between the radiation receiver and a final element of        the radiation detector.

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising:

-   -   projecting a patterned beam of radiation onto a target portion        of a substrate; and    -   projecting a beam of radiation onto a sensor at substrate level        that receives the beam of radiation via a radiation receiver and        detects the beam of radiation via a radiation detector, the        sensor being arranged to avoid loss of radiation between the        radiation receiver and the final element of the radiation        detector.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts a another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts an ILIAS sensor module according to the prior art;

FIG. 6 depicts an ILIAS sensor module with an elongated transmissiveplate according to an embodiment of the present invention;

FIG. 7 depicts an ILIAS sensor module with a filler sheet according toan embodiment of the present invention;

FIGS. 8 a and 8 b depict a luminescence based DUV TIS according to theprior art;

FIG. 9 depicts schematically an arrangement of filler sheets in a stackcomprising transmissive plate, luminescence layer and photodiodeaccording to an embodiment of the present invention, along with exampleray paths;

FIG. 10 depicts schematically two arrangements of a filler sheetsandwiched between a luminescence layer and photodiode according to anembodiment of the present invention, along with example ray paths;

FIGS. 11 a and 11 b depict a non-luminescence based DUV TIS with afiller sheet according to an embodiment of the present invention;

FIG. 12 depicts the use of a filler sheet in a stack comprising atransmissive plate and a photodiode, along with example ray paths;

FIG. 13 depicts a sensor at substrate level, comprising a photocell incombination with a luminescence layer, with a filler sheet positionedabove the luminescence layer;

FIG. 14 depicts a sensor at substrate level, comprising a photocellpositioned immediately below the radiation receiver;

FIG. 15 depicts a sensor at substrate level, comprising a photocell incombination with a luminescence layer positioned immediately below theradiation receiver;

FIG. 16 depicts a sensor at substrate level, comprising a photocell incombination with a luminescence layer positioned immediately below thequartz sensor body;

FIG. 17 depicts a sensor at substrate level, comprising a photocell incombination with a luminescence layer and a diffractive lens; and

FIG. 18 depicts a sensor at substrate level, comprising a photocell incombination with a luminescence layer and a micro-lens.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   projection system (e.g. a refractive projection lens system) PL        configured to project a pattern imparted to the radiation beam        PB by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam PB passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam PB. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam PB, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke-module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIGS. 5 to 18 depict improved substrate-level sensors according toembodiments of the invention. These sensors comprise a radiationreceiver (2,18) and a radiation detector (8,24,40). In an embodiment,exposure radiation is directed from the final element of the projectionsystem PL through an immersion liquid 1 at least partly filling a spacebetween the final element of the projection system PL and the substrateW. The detailed configuration of each of these elements depends on theproperties of the radiation to be detected. The sensor at substratelevel may comprise a photocell only, for use in cases where it isdesirable for the photocell to receive the radiation directly.Alternatively, the sensor at substrate level may comprise a luminescencelayer in combination with a photocell. In this arrangement, radiation ata first wavelength is absorbed by the luminescence layer and reradiateda short time later at a second (longer) wavelength. This arrangement isuseful, for example, where the photocell is designed to work moreefficiently at the second wavelength.

The radiation receiver (2,18), which may be a layer with a pinhole, agrating or another diffractive element fulfilling a similar function,may be supported on top of a quartz sensor body 20, i.e. on the sameside of the body as the projection system. The radiation detector(8,24,40), in contrast, may be arranged within the sensor body 20, orwithin a concave region formed on the side of the sensor body 20 facingaway from the projection system.

At boundaries between media of different refractive indices, aproportion of incident radiation will be reflected and potentially lostfrom the sensor. For optically smooth surfaces, the extent to which thisoccurs depends on the angle of incidence of the radiation and thedifference in refractive index of the media in question. For radiationincident at and above a “critical angle” (conventionally measured fromnormal incidence) total internal reflection may occur, leading toserious loss of signal to later elements of the sensor. This may be aparticular problem in high NA systems where radiation may have a higheraverage angle of incidence. In an embodiment of the present invention,an arrangement is provided whereby gas (e.g., air) is excluded from theregion between the radiation receiver (2,18) and the radiation detector(8,24,40) in order to avoid interfaces between media of high refractiveindex and the gas.

In addition to losses due to partial and total internal reflection,absorption may also seriously reduce the intensity of radiation reachingthe photocell, as may scattering from interfaces that are not opticallysmooth.

A substantial contribution to reduced sensitivity of a sensor mayinclude loss of radiation from the sensor before it even reaches thefinal element of the radiation detector. As discussed above, radiationmay be lost due to scattering from rough surfaces or via total orpartial internal reflection at interfaces within the detector.Alternatively, gas gaps containing oxygen and water may lead tosubstantial absorption of radiation passing therethrough.

For example, FIG. 5 shows an WLIAS sensor module according to the priorart. This module has a shearing grating structure 2 as radiationreceiver, supported by a transmissive plate 4, which may be made ofglass or quartz. A quantum conversion layer 6 is positioned immediatelyabove a camera chip 8 (the radiation detector), which is in turn mountedon a substrate 10. The substrate 10 is connected to the transmissiveplate 4 via spacers 12 and bonding wires 14 connect the radiationdetector to external instrumentation. An gas gap is located between thequantum conversion layer 6 and the transmissive plate 4. In a setup suchas this designed for 157 nm radiation, for example, the gas gap withinthe sensor cannot easily be purged so that it may contain significantproportions of oxygen and water, which absorb radiation. Signal istherefore lost and the effect becomes worse for larger angles as thesehave a longer path length through the gas. Thus, the dynamic rangerequirements for the sensor become more severe.

According to an aspect of the invention, the sensor at substrate levelmay comprise one or more transmissive filler sheets. These sheets may bepositioned within the sensor so as to remove a gas gap between theradiation receiver (2,18) and the final element of the radiationdetector (8,24,40). Alternatively or additionally, the transmissiveplate may be arranged to extend continuously between the radiationreceiver (2,18) and the radiation detector (8,24,40), thus avoiding anygas gap in this region. This approach may reduce the need for additionalfiller sheets and associated interfaces.

For example, FIGS. 6 and 7 show improved ILIAS sensor modules accordingto embodiments of the invention. In FIG. 6, the gas gap has been removedby changing the shape of the transmissive plate 4 to fit directly to theradiation detector 8. This arrangement is made more difficult by theneed to provide access for the bonding wires 14 and necessitates anelongated form. From an engineering point of view, the alternativearrangement using one or more transmissive filler sheets shown in FIG. 7is easier to realize. Here, a filler sheet 16 of the same material asthe transmissive plate 4, or of similar optical properties, is insertedbetween the transmissive plate 4 and the quantum conversion layer 6. Theremoval of the gas gap reduces transmission losses and relaxes dynamicrange requirements (or, alternatively speaking, improves the effectivedynamic range). Both arrangements improve refractive index matching andreduce the extent of spurious internal reflections at the interface withthe transmissive plate 4.

The material for each filler sheet may be chosen to be highlytransmissive for the predominant wavelength of radiation that will passthrough it. Immediately following the radiation receiver (2,18) theradiation wavelength may be extremely short (e.g. 157 nm), butluminescence layers (22) occurring later in the sensor may emit longerwavelength radiation in which case it would be advantageous to choosedifferent materials for filler sheets in the respective regions.

The material for each filler sheet may further be chosen to providerefractive index matching with media with which it is in contact. Forexample, the refractive index of air is very different from typicalsensor components leading to strong partial reflection and a smallercritical angle for total internal reflection. By providing a fillersheet with a refractive index closer to the component in questioninstead of a gas gap, partial reflection is reduced and the criticalangle for total internal reflection is increased. This feature has theeffect of further improving the effective dynamic range of the sensor.

The filler sheet(s) may be positioned in contact with optically roughcomponent interfaces and treated so as to follow the contours of thesurface roughness. Where sensor elements have been machined they willnormally have a surface roughness on the length scale of incidentradiation. When a significant refractive index mismatch is present atsuch a surface, a significant proportion of the incident radiation willinevitably be lost due to scattering at the surface. By using a fillersheet(s) and treating it so that it follows the contours of the surfaceroughness (and thereby purge any gas that may exist there) the smallerdiscontinuity in refractive index reduces the extent of radiation lossat the interface.

FIG. 8 a shows a DUV transmission image sensor according to the priorart. FIG. 8 b shows a magnified view of the processing element forclarity. The pattern of transmissive grooves 18, constituting theradiation receiver in this case, is realized by means of e-beamlithography and dry etching techniques in a thin metal layer depositedon a substrate by means of sputtering. Any DUV light that is projectedtowards the grooves 18 is transmitted by the transmissive plate 4 (whichmay be quartz or fused silica) and hits the underlying luminescentmaterial 22, or “phosphor”. The luminescent material 22 may consist of aslab of crystalline material that is doped with rare-earth ions, e.g.yttrium-aluminum-garnet doped with cerium (YAG:Ce). The main purpose ofthe luminescent material 22 is to convert the DUV radiation into moreeasily detectable visible radiation, which is then detected by thephotodiode 24. DUV radiation that has not been absorbed and convertedinto visible radiation by the phosphor 22 may be filtered out before itreaches the photodiode 24 by a filter 26, e.g. a BG-39 or UG filter.

In the above arrangement, gas may be present in the gaps betweencomponents mounted in the sensor housing 25, yielding a number ofgas/material/gas interfaces that interrupt the propagation of radiation.By considering the path of DUV radiation and radiation arising fromluminescence, it is possible to identify regions where radiation islikely to be lost. The first region of interest is the rear-side 28 ofthe transmissive plate 4, reached by DUV radiation after it has passedthrough the grooves 18 and transmissive plate 4. Here, the surface hasbeen formed by mechanical means, such as by drilling, and is inevitablyrough on the scale of the wavelength of the radiation. Radiation maytherefore be lost due to scattering, either back into the transmissiveplate 4 or out past the luminescent material 22. Secondly, after thisinterface, the DUV light encounters the optically smooth gas/YAG:Ceinterface, where a substantial amount of reflection may occur due to therefractive index mismatch, particularly in systems of high NA. Thirdly,the luminescent material 22 emits radiation in random directions. Due toits relatively high refractive index, the critical angle for totalinternal reflection at the YAG:Ce/air boundary is around 33° (wherethere is air in the gap between the YAG:Ce and the filter) from thenormal, meaning that a large proportion of radiation incident on theboundary is reflected out of the system and lost through the side wallsof the luminescent material 22. Finally, the part of the luminescencethat is directed towards the photodiode has to overcome the gas/quartzinterface on the diode surface where surface roughness may again accountfor loss of detected signal.

FIGS. 9 and 10 illustrate schematic arrangements that address theproblems described above as well as exemplary radiation ray paths. Oneor more filler sheets 30, which may be made from light transmittingplastics, are inserted between components to reduce the effect ofradiation scattering at gas/material interfaces with high surfaceroughness or large refractive index discontinuities. For example, thefiller sheet(s) 30 may be arranged to be transmissive for either DUVradiation, visible radiation, or both. Additionally, the refractiveindex of each filler sheet 30 may be tuned to provide the most efficientrefractive index matching between media with which it is in contact.Where a filler sheet 30 is in contact with an optically rough surface,some deformation of the filler sheet 30 may be necessary to ensure thatit closely follows the surface roughness and does not leave any tiny gaspockets. This may be achieved by mechanically compressing the fillersheet 30 onto the surface in question. Alternatively or additionally,the filler sheet 30 may be gently heated (avoiding excessive oxidationor other chemical decomposition that may occur at high temperature)until it flows sufficiently to follow the surface roughness. It is alsopossible to use a fluid as a filler sheet, chosen to have as high arefractive index as possible, for example Fomblin perfluorinatedpolyether.

The filler sheet(s) may be arranged to have a refractive index equal toor greater than the refractive index of the immersion liquid. In thetypical case where the relevant interfaces (immersion liquid totransmissive plate and transmissive plate to filler sheet) are parallelto each-other and perpendicular to the axis of the projection system,this condition ensures that no internal reflection will occur at thetransmissive plate to filler sheet interface. If it were required tomake these interfaces non-parallel then a corresponding increase in thelower refractive index bound for the filler sheet 30 may be chosen.

FIG. 9 shows a possible implementation of the filler sheet(s) 30 in aDUV sensor comprising a transmissive plate 4, luminescence layer 22 andphotodiode 38. The right-hand diagram comprises filler sheets 30 while,for comparison, the left-hand diagram does not. In each case, arrowsshow exemplary ray paths through the stack, with internal reflectionoccurring at the YAG:Ce/diode interface when the filler sheets areabsent.

The filler sheet 30 may either consist of a single sheet, as shown inthe left-hand diagram of FIG. 10, for example, or of a composite sheetconsisting of two or more layers of different refractive index with amicro-lens array pattern 34 formed at the boundary between the twolayers.

According to this embodiment, the filler sheet not only acts to improverefractive index matching and reduce absorption but focuses rays so asto reduce their angle to the normal and thereby improve transmission atlater interfaces.

One or more of the optical components (e.g. transmissive plate, fillersheet and/or luminescence layer) of the sensor at substrate level maycomprise an internal-reflection-enhancing layer on its outer lateralsurface. This layer may be constructed by roughening the outer surfaceand/or applying a metallic layer to it. This feature acts to reflectradiation back into the sensor that would otherwise have been lost.

The measures discussed above significantly improve the signal-to-noiseperformance of TIS type sensors, a factor likely to become increasinglycritical due to-the trend towards designs with ever decreasing linewidths of the grooves 18 in the radiation receiver. In addition to thelosses associated with internal sensor interfaces, a large proportion ofthe signal may also be lost due to inefficient conversion of the DUVradiation to visible radiation within the luminescence layer 22.According to a further embodiment of the invention, the luminescencelayer 22 is removed from the sensor and DUV radiation is arranged toimpinge directly onto a suitably adapted photodiode 40. Photodiodesprovide a shorter path from photon to electron (i.e. from radiation tosignal) and diodes sensitive to DUV may be obtained with arbitrary shapeand size. Such diodes 40 may be capable of detecting DUV radiation downto 50 nm wavelength with a conversion efficiency of the order of 0.20A/W. Long diode lifetimes are achieved by depositing oxy-nitridepassivation layers on the diode entrance windows. The arrangement isillustrated schematically in FIG. 11 a. FIG. 11 b shows a magnified viewof the processing element for clarity. Here, photodiodes 40 arepositioned below the grooves 18 in such a way that incident radiationonly has to propagate through the grooves 18, a transmissive plate 4,and a refractive index matching filler sheet 30 (FIG. 11 b shows twopossible variations on the filler sheet, a homogeneous layer (top) and adual layer with micro-lens patterning (bottom)) in order to reach thephotodiode 40. For the refractive index matching filler sheet 30, aliquid dielectric resist HSQ may be used based on its quartz-likeproperties after moderate temperature curing. This may provide anoptimal refractive index match. The diodes may be electronicallymonitored via the rear connections to facilitate maintenance andeventual replacement.

The path of radiation beams through the above arrangement is illustratedin FIG. 12. The right-hand diagram comprises a filler sheet 30 while,for comparison, the left-hand diagram does not.

FIGS. 13 to 18 depict further embodiments of the invention, whereinlight (arrows) propagates from the final element of the projectionsystem PL through the immersion liquid 1 onto the sensor. In FIGS. 13 to16, components are arranged to remove low refractive index parts fromthe sensor.

In the embodiment according to FIG. 13, the sensor comprises aluminescence layer 22 in combination with a photocell 24. A filler sheet30 is arranged between the luminescence layer 22 and the radiationreceiver 2, in such a way as to avoid interfaces with gas between thoseelements. The purpose of the filler sheet is to increase the amount oflight continuing through to the detector. A gas gap 3 is arrangedbetween the luminescence layer 22 and the photocell 24, which isdescribed below.

In the embodiment according to FIG. 14, the sensor comprises a photocell40, which is arranged to be in contact with the radiation receiver onthe opposite side to the projection system. This arrangement avoids allinterfaces with gas. Absorption may also be reduced because theradiation does not pass through intermediate layers.

In the embodiment according to FIG. 15, a sensor arrangement analogousto that shown in FIG. 13 is depicted. However, an extended luminescencelayer 22 is used to fill the space between the front and rear of thesensor body 20. Interfaces and interface-induced reflections are therebyavoided. A gas gap 3 is arranged between the luminescence layer 22 andthe photocell 24, which is described below.

FIG. 16 depicts a further embodiment of the invention, wherein thesensor arrangement comprises a luminescence layer 22 in combination witha photocell 24. In this case the luminescence layer may take a flattenedform and be located on the side of the sensor body 20 facing away fromthe projection system and in contact with the sensor body 20, thusavoiding interfaces with gas before the luminescence layer. A gas gap 3is arranged between the luminescence layer 22 and the photocell 24,which is described below.

FIG. 17 depicts an embodiment that comprises a diffractive lens 30located between the radiation receiver 2 and the radiation detector 24.The diffractive lens 30 acts to focus the incident radiation bydiffraction towards the luminescence material 22, thereby improving theability of the detector to accept rays that are incident at high angles(such as in systems with high NA). The use of a diffraction-basedmechanism allows the lens to be constructed in a miniature form. Analternative and/or additional approach is depicted in FIG. 18, wherein amicro-lens 40 (operating principally by refraction rather thandiffraction) is included in an equivalent position to the diffractivelens 30. In the particular arrangement shown, the micro-lens 40 isformed directly from the material of the sensor body 20. Thisarrangement avoids having to add the lens as a separate component, whichincreases system complexity, and also avoids the problem of additionalsignal loss that may occur due to reflection at interfaces with the lensmaterial. However, a different material may also be used for themicro-lens 40 without departing from the scope of the invention.

In embodiments comprising a luminescence layer 22 in combination with aphotocell 24, a small gas gap (of the order of microns) may be arrangedbetween the luminescence layer 22 and the photocell 24. The small sizeof the gap means that even high angle rays that refract to higher anglesat the gas interface may still impinge on the photocell without makingthe photocell overly large. In addition, some proportion of radiationarriving at the gas interface above the critical angle may stillpropagate to the photocell via tunneling of the evanescent wave acrossthe gas gap. In an embodiment, the size of the gap is smaller than thewavelength of incident radiation.

The radiation receiver may comprise a grating and or an element having apinhole, depending on the function of the sensor.

The sensors may be located at the level of the substrate and inparticular such that the radiation receiver 2,18 is at substantially thesame distance from the final element of the projection system as thesubstrate W.

Another solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The seal member is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the seal member and the surfaceof the substrate. In an embodiment, the seal is a contactless seal suchas a gas seal. Such as system with a gas seal is disclosed in U.S.patent application Ser. No. 10/705,783, hereby incorporated in itsentirety by reference.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

The present invention can be applied to any immersion lithographyapparatus, in particular, but not exclusively, those types mentionedabove.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The-substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. An exposure apparatus comprising; a projection optical system forprojecting a pattern on a reticle onto a substrate; a light-receiver atsubstantially substrate level; a detector below the light-receiver forreceiving light from the light-receiver; and a structure or liquid,arranged between said light-receiver and the detector, to increase thecritical angle of acceptance of the light between the light-receiver andthe detector than would otherwise without the structure or liquid, thestructure or liquid directly contacting the light-receiver, or thedetector, or both the light-receiver and the detector.
 2. An exposureapparatus according to claim 1, wherein said exposure apparatus is animmersion type exposure apparatus that immerses at least part of saidprojection optical system in a fluid that has a refractive index of 1 orgreater.
 3. A projection exposure apparatus which has a projectionoptical system and projects a pattern onto a substrate through saidprojection optical system, said apparatus comprising: a sensor unitwhich comprises a detector for detecting light incident through saidprojection optical system, and a light-receiver which transmits theincident light and blocks the detector from a fluid on a side of thelight-receiver opposite to the detector, wherein a space between saidlight-receiver and said detector is filled with a medium having arefractive index which is greater than 1, the medium directly contactingthe light-receiver, or the detector, or both the light-receiver and thedetector.
 4. An apparatus according to claim 3, wherein said sensor unitis arranged not to include an interface at which total reflection of theincident light entering said sensor unit occurs.
 5. An apparatusaccording to claim 3, wherein the medium comprises a liquid.
 6. Anapparatus according to claim 5, wherein the liquid is an inert liquid.7. An apparatus according to claim 5, wherein the liquid includesperfluorinated polyether.
 8. An apparatus according to claim 3, whereinsaid sensor unit comprises a member covering said detector.
 9. Anapparatus according to claim 8, wherein said member comprises one of aplastic and glass.
 10. An apparatus according to claim 3, wherein saidsensor unit comprises a light-shielding member with a predeterminedshape at an incident portion for the incident light.
 11. An apparatusaccording to claim 3, wherein the light-receiver comprises apredetermined light-shielding pattern.
 12. A device manufacturing methodcomprising: projecting a pattern onto a substrate using a projectionexposure apparatus which has a projection optical system and projects apattern onto a substrate through said projection optical system, saidprojection exposure apparatus comprising a sensor unit which comprises adetector for detecting light incident through said projection opticalsystem, and a light-receiver which transmits the incident light andblocks the detector from a fluid on a side of the light-receiveropposite to the detector, wherein a space between said light-receiverand said detector is filled with a medium having a refractive indexwhich is greater than 1; and developing the substrate onto which thepattern has been projected.
 13. A sensor unit comprising: a detectorwhich detects light; and a light-receiver which transmits light incidentthereon and blocks the detector from a fluid on a side of thelight-receiver opposite to the detector, wherein a space between saidlight-receiver and said detector is filled with a medium having arefractive index which is greater than 1, the medium directly contactingthe light-receiver, or the detector, or both the light-receiver and thedetector.
 14. A unit according to claim 13, wherein said unit isarranged not to include an interface at which total reflection of theincident light entering the unit occurs.
 15. A unit according to claim13, wherein the medium comprises a liquid.
 16. A projection exposureapparatus which has a projection optical system and projects a patternonto a substrate through said projection optical system, said apparatuscomprising: a sensor unit which comprises a detector which detects lightincident through said projection optical system, and a light-receiverwhich transmits the incident light and blocks the detector from a fluidon a side of the light-receiver opposite to the detector, wherein aspace between said light-receiver and said detector is filled with aliquid.
 17. A device manufacturing method comprising: projecting apattern onto a substrate using a projection exposure apparatus which hasa projection optical system and projects a pattern onto a substratethrough said projection optical system, said projection exposureapparatus comprising a sensor unit which comprises a detector whichdetects light incident through said projection optical system, and alight-receiver which transmits the incident light and blocks thedetector from a fluid on a side of the light-receiver opposite to thedetector, wherein a space between said light-receiver and said detectoris filled with a liquid; and developing the substrate onto which thepattern has been projected.
 18. A sensor unit comprising: a detectorwhich detects light; and a light-receiver which transmits the incidentlight and blocks the detector from a fluid on a side of thelight-receiver opposite to the detector, wherein a space between saidlight-receiver and said detector is filled with a liquid.
 19. Alithographic apparatus comprising: a projection optical systemconfigured to project a pattern onto a substrate; a liquid supply systemconfigured to provide a liquid between the projection optical system andthe substrate; and a sensor unit including a radiation receiver, aradiation detector configured to detect radiation transmitted throughthe projection optical system, and a plastic sheet covering theradiation detector and located between the radiation receiver and theradiation detector, the plastic sheet directly contacting the radiationreceiver, or the radiation detector, or both the radiation receiver andthe radiation detector, wherein the radiation detector is configured todetect radiation transmitted through the projection optical system, theliquid, the radiation receiver and the plastic sheet.